WEDNESDAY, Dec. 5 (HealthDay News) --Taking the
breast cancer drug tamoxifen for a decade, instead of the standard five years, further reduces the long-term chances of recurrence and risk of dying from the disease, new British research suggests.

The increase in benefit is especially noticeable after the 10th year, said study author Richard Peto, a professor of medical statistics and epidemiology at the University of Oxford.

Tamoxifen is widely used for treating estrogen receptor or ER-positive breast cancer in women who are pre-menopausal. ER-positive cancers need estrogen to grow, and the drug blocks the activity of estrogen in the breast.

Currently, tamoxifen is usually given daily for five years after a cancer is treated. Doctors have known that five years is more effective than two, and that it reduces the rate of death from breast cancer by about a third in comparison to not taking it, and that it does so for 15 years after the diagnosis.

In the new study, the researchers wanted to see if longer treatment would be even better. They compared results in women who took tamoxifen for five years and 10 years.

A decade was better.

"It's been one moderate therapeutic advance after another," Peto said.

The new research is to be presented Wednesday at the San Antonio Breast Cancer Symposium and published simultaneously in The Lancet.

In the new study, Peto's team looked at nearly 13,000 women who had already taken the drug for five years.

The women were assigned to continue the drug for another five years or to stop taking it. For the analysis, the researchers focused on the 6,846 women who had ER-positive breast cancer, as the main benefit of tamoxifen is thought to be for them. Of these, about half kept taking the drug and half did not.

After about eight years of follow-up, there were 617 recurrences in the drug group and 711 in the group that had stopped. While 331 in the drug group died of breast cancer during follow up, 397 in the group that stopped died.

The risk of breast cancer death five to 14 years after the diagnosis was slightly over 12 percent among those who stayed on the drug versus 15 percent among those who stopped.

The follow-up is continuing, Peto said. "We are going to follow for another five years, to see what happens," he said. "I think there will probably be future gain."

Will tamoxifen end up being a lifetime drug, once it is prescribed? "No, because tamoxifen has side effects," Peto explained.

The side effects are well documented -- an increased risk of cancer of the uterus and blood clots in the lungs are major ones. In the study, those who continued on the drug had higher risks of both conditions. However, the risks were outweighed by the reduction in death risk, the researchers added.

The research will be practice-changing, said Dr. Joann Mortimer, director of the Women's Cancers Program at City of Hope Comprehensive Cancer Center, in Duarte, Calif.

"Whether women will allow it to change practice remains to be seen," she said, referring to the reluctance of women to take anti-cancer drugs like tamoxifen. Compliance is "atrocious," she said.

Women stop the drug for many reasons, she said, including side effects.

Even so, she said, the new research is reason enough for women who may benefit from tamoxifen to talk to their doctor about it, Mortimer said.

The study was funded by AstraZeneca, which makes tamoxifen, as well as Cancer Research U.K., Medical Research Council, the U.S. Army and the European Union.